Al-Si based alloy and alloy member made therefrom

ABSTRACT

An Al—Si based alloy and an alloy member made of the alloy, in which when alloys produced by diecasting under high vacuum conditions are welded, weldability can be improved without increasing plate thickness of welded portions and reducing gas content in diecasting.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an Al—Si based alloy and to an alloy member made of the Al—Si based alloy, and in particular, relates to a technique for producing an Al—Si based alloy which is produced by diecasting under a high vacuum and has good weldability.

2. Related Art

Until now, various techniques have been disclosed as production methods for casting by diecasting of aluminum. For example, in order to improve mechanical properties, in particular, elongation by aging only, without solution heat treatment, Japanese Unexamined Patent Application Publication No. 7-91624 discloses a production method of aluminum alloy casting including a step for forming a casting by diecasting aluminum alloy, a step for coating on the casting, and a step for heating which bakes the coating on the coated casting and aging the casting at the same time, wherein the aluminum alloy includes, by weight, Si at 7.5 to 9.5%, Cu at 0.1 to 0.3%, Mg at 0.1 to 0.32%, Fe at 0.5 to 0.9%, Mn at 0.2 to 0.6%, and Sr at 0.03 to 0.05%, and the balance consisting of Al. In addition, in order to improve dimensional accuracy and toughness of the diecasting products, Japanese Unexamined Patent Application Publication No. 9-3610 discloses a production technique of aluminum diecasting products including a step for diecasting aluminum molten metal including Si at 5 to 13%, Mg at 0.5% or less, Mn at 0.1 to 1.0%, and Fe at 0.1 to 2.0%, a step of heating the diecasting product to 400 to 500° C., and a step for cooling to room temperature at a cooling rate of 10° C./sec or less, so that average particle size of the diecasting product is 20 μm or less.

Of diecasting products produced by these disclosed techniques, diecasting products using for example, 365 alloy according to the AA standard, etc., has superior toughness, and moreover, fluidity is superior because the products have uniform alloy which exists in a region from the Al—Si hypo eutectic region to the eutectic region. However, the products made of these alloys do not exhibit superior weldability.

That is, in the case in which conventional Al—Si based alloys obtained by diecasting are welded, welded metal flows out, and sufficient weld bead dimensions such as throat thickness and leg length are not obtained. Here, the above throat thickness and leg length will be explained. FIG. 1 is a side view showing a welding state of alloy members consisting of Al—Si based alloy. As shown in the figure, the throat thickness is the largest height 2 in the protruding portion of weld metal 1 (shaded portion) which exists in a welded portion, and the leg length is lengths 3 and 4 in each direction of the contact portion between the base metal (each Al—Si based alloy) and weld metal 1. When the throat thickness 2 is not ensured sufficiently, volume of the weld metal 1 is small, and when the leg lengths 3 and 4 are not ensured sufficiently, contact area between the weld metal 1 and the base metal is small. Therefore, in either case, strength of the welded portion after welding is not sufficiently obtained.

In addition, in producing the conventional Al—Si based alloys, superior quality and strength of the weld bead are not obtained when Si content in the alloys is large, even if diecasting is carried out under a vacuum so as to decrease gas content. The reason for this is that hydrogen gas contained in diecasting members bubble out and then a blowhole is formed in the weld bead by generated and accumulated hydrogen gas bubble. For example, when the Si content in the alloy is large, even if diecasting is carried out under a vacuum so as to decrease the hydrogen content to 5.0 cc/100 g, crystallization of solid phase in welded portions is delayed and bubbling of microbubbles of gas is thereby caused due to dissolving, generating and accumulating of hydrogen gas. As a result, a blowhole is formed in the weld bead and therefore, superior strength and quality of the alloy, etc., are not obtained. In view of such circumstances, in order to obtain sufficient strength until now in the welded portion, it was necessary that plate thickness of the welded portion zone be increased or that gas content in diecasting be reduced.

DISCLOSURE OF INVENTION

The present invention was completed in view of the above circumstances, and objects of the present invention are therefore to provide an Al—Si based alloy and an alloy member made of the alloy, in which when alloys produced by diecasting under high vacuum conditions are welded, weldability can be improved without increasing plate thickness of welded portions and gas content is reduced in diecasting.

The present inventors researched Al—Si based alloys and alloy members made of the alloy, which is produced by diecasting under high vacuum conditions and exhibits superior weldability, as described above. As a result, it was concluded that according to the Al—Si based alloy, superior weld beads can be formed and preferable weld strength, that is, sufficient weldability, can be thereby obtained, by the following known facts shown in 1) to 4).

1) FIG. 2 is a graph showing the relationship between the liquid phase rate and temperature in welding in connection with each Si content in an Al—Si based alloy. As shown in the figure, in welded metal immediately after welding, the lower the Si content, the higher the starting temperature of crystallization of the solid phase, and the viscosity of the welded metal after welding is increased with increasing of solid phase. Therefore, for example, when the Si content in the Al—Si based alloy is controlled to within 7.5 to 9.0 mass %, viscosity in melting can be sufficiently ensured, and solidifying time of the weld bead can be shortened by increasing the liquid temperature and the solid temperature. As a result, throat thickness and leg length can be sufficiently ensured by preventing flowing out of weld metal, and bubbling of microbubbles of gas due to dissolving, generating and accumulating of hydrogen gas is prevented by crystallization of solid phase and blowhole in the weld bead can also thereby be prevented from forming. Therefore, weldability can be improved.

2) When Mu, Cr, Ti, and Sr are contained in a small amount as a improvement processing agent, the alpha phase of aluminum in solidifying the weld bead is made finer, the beta phase of Si is fine-spheroidized, and therefore, strength of the welded portion can be improved.

3) When Mn, Cr, and Fe are contained in the Al—Si based alloy, the alloy can be prevented from adhering to and burning in the metal mold, even if molten metal temperature, casting speed, and casting pressure are increased in order to compensate for fluidity in diecasting.

4) It is preferable that Ti not be contained as a constituent element of the alloy in order to attempt to improve toughness.

The present invention was completed in view of the above knowledge.

That is, an Al—Si based alloy according to the present invention includes Si at 7.5 to 9.0 mass %, Mg at 0.2 to 0.4 mass %, Mn at 0.3 to 0.5 mass %, Cu at 0.03 to 0.2 mass %, Fe at 0.1 to 0.25 mass %, Sr at 0.005 to 0.02 mass %, and the balance consisting of Al and inevitable impurities.

It is preferable that the Al—Si based alloy include Si at 7.5 to 8.5 mass %, Mg at 0.2 to 0.3 mass %, and Mn at 0.3 to 0.4 mass %.

As shown in the above, according to the present invention, an Al—Si based alloy which exhibits superior weldability in diecasting under high vacuum conditions can be obtained by optimizing of contents of Si, Mg, Mn, Cu, Fe, and Sr which are constituent elements in the alloy. In welding of alloy members produced by using the alloy of the present invention, by improving the weldability, plate thickness of welded portion in the alloy members is decreased and length of weld bead is shortened, and the alloy members obtained by diecasting can be thereby made lighter. In addition, the alloy members of the present invention are preferably employed as members for various processing since toughness is high as that of conventional alloy members and weldability is also high. Furthermore, welding processes can be rationalized in welding of the alloy members of the present invention, since plate thickness can be decreased as described above, etc. Additionally, diecasting process can also be rationalized since the contained gas amount in diecasting is increased.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view showing a welding state of alloy members consisting of an Al—Si based alloy.

FIG. 2 is a graph showing the relationship between the liquid phase rate and temperature in welding in connection with each Si content in the Al—Si based alloy.

FIGS. 3A to 3C are graphs showing mechanical properties in the case in which molten metal of each Al—Si—Mg alloy in which Mg content was set to a desired value and Si content was changed was cast in a metal mold for an ASTM test piece by a 200-ton diecasting machine, respectively, and then a test piece for tension test was processed, and subsequently, a tension test was carried out without heating. FIG. 3A shows tensile strength, FIG. 3B shows proof stress, and FIG. 3C shows elongation.

FIGS. 4A to 4C are graphs showing mechanical properties in the case in which molten metal of each Al—Si—Mg alloy in which Si content was set to a desired value and Mg content was changed was cast in a metal mold for an ASTM test piece by a 200-ton diecasting machine, respectively, and then a test piece for tension test was processed, and subsequently, a tension test was carried out without heating. FIG. 4A shows tensile strength, FIG. 4B shows proof stress, and FIG. 4C shows elongation.

FIG. 5 is a plane view showing a test piece for tension test cut down from each plate-shaped diecasting product (Examples 1 to 5 and Comparative Examples 6 to 12 of the present invention).

FIG. 6 is a side view showing a point of welding and a point of measurement of throat thickness and blowhole in the Examples.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, reasons for the limitation of the contents of each constituent element in the Al—Si based alloy according to the present invention will be explained.

FIGS. 3A to 3C are graphs showing the relationships of tensile strength, proof stress and elongation in the case in which molten metal of each Al—Si—Mg alloy in which Mg content was set to a desired value and Si content was changed was cast in a metal mold for an ASTM test piece by a 200-ton diecasting machine, respectively, then a test piece for tension test was processed, and subsequently, a tension test was carried out without heating. As is apparent from the results of FIG. 2 and FIGS. 3A and 3B, when the Si content is 7.5 mass % or more, fluidity of molten metal in diecasting is superior and mechanical properties such as tensile strength, proof stress, etc., are thereby superior. In the meantime, the following conclusions were obtained from the results of FIG. 2 and FIG. 3C. That is, when the Si content is 9.0 mass % or less, the solid phase in the welded portion after welding is crystallized at a high temperature range and viscosity of the welded portion after welding is increased with increase of the solid phase, and the weld metal is prevented from flowing out. As a result, throat thickness and leg length are sufficiently ensured, bubbling of microbubbles of gas is prevented by the crystallization of the solid phase, and formation of blowhole in weld bead is prevented also. Therefore, toughness such as elongation can be sufficiently ensured. As described above, the Si content is set to be 7.5 to 9.0 mass %. Here, it is preferable that Si content be 8.5 mass % or less, since elongation can be ensured at a higher level.

FIGS. 4A to 4C are graphs showing the relationships of tensile strength, proof stress and elongation in the case in which molten metal of each Al—Si—Mg alloy in which Si content was set to a desired value and Mg content was changed was cast in a metal mold for an ASTM test piece by a 200-ton diecasting machine, respectively, then a test piece for tension test was processed, and subsequently, tension test was carried out without heating. Mg is contained in order to improve mechanical properties such as tensile strength and proof stress, as well as Si. As is apparent from FIGS. 4A to 4C, when Mg content is less than 0.2 mass %, the improving effect of the above mechanical properties is low. In contrast, when the content exceeds 0.4 mass %, toughness such as elongation is decreased. Therefore, the Mg content is set to be 0.2 to 0.4 mass %. Here, it is preferable that Mg content be 0.3 mass % or less, since elongation can be ensured at a higher level.

In the following, tensile strength, proof stress and elongation in tensile test in the case of the above Si content and Mg content will be explained in detail. That is, as is apparent from FIG. 3A and FIG. 4A, the tensile strength is easily affected by the Mg content more than the Si content. When the Si content is 9.0 mass % or 10.0 mass %, the tensile strength increases in proportion to Mg content; however, when the Si content is less than 7.5 mass %, the tensile strength is rapidly lowered. In order to maintain the tensile strength, it is preferable that the Si content be 7.5 mass % or more and the Mg content be 0.2 mass % or more. In addition, as is apparent from FIG. 3B and FIG. 4B, the proof stress is also easily affected by the Mg content more than of the Si content. When the Si content is 9.0 mass % or 10.0 mass %, superior proof stress is exhibited; however, there is less difference of tensile strength than that of proof stress between these contents and there is less difference when the Si content is 7.5 mass % or more. Furthermore, as is apparent from FIG. 3C and FIG. 4C, the elongation is easily affected by not only the Mg content but also the Si content. That is, the elongation tends to be in inverse proportion to the Si content, and the less the Si content, the larger the elongation. In addition, for elongation it is preferable that the Mg content be within 0.2 to 0.4 mass %.

Next, Mn is contained in order to suppress that toughness such as elongation is decreased by Fe compound which is a needle coarse crystal deposited in diecasting. When the Mn content is less than 0.3 mass %, the above effect in which decreasing of the toughness is suppressed is poor. In contrast, when the content exceeds 0.5 mass %, the above suppressing effect is saturated and it is difficult to obtain further effect. Therefore, the Mn content is set to be 0.3 to 0.5 mass %. Here, it is preferable that the Mn content be 0.4 mass % or less, since intermetallic compound is prevented from being produced and the elongation can be sufficiently ensured.

In addition, Cu is contained in order to improve tensile strength and proof stress. When Cu content is less than 0.03 mass %, high purity Al mother alloy must be used in diecasting, and in addition, the cleanliness of the fusion furnace and the holding furnace must be more precisely controlled than those of conventional methods, and producing cost is increased. In contrast, when the content exceeds 0.2 mass %, by affecting the Si content, toughness such as the elongation lowering and moreover, corrosion resistance are deteriorated. Therefore, the Cu content is set to be 0.03 to 0.2 mass %.

Furthermore, Fe content is less than 0.1 mass %, high purity Al mother alloy must be used in diecasting, and in addition, the cleanliness of fusion furnace and holding furnace must be more precisely controlled than those of conventional methods, and producing cost is increased. In contrast, when the Fe content exceeds 0.25 mass %, Fe compound is deposited as a needle coarse crystal in diecasting, and toughness such as elongation is decreased. Therefore, the Fe content is set to be 0.1 to 0.25 mass %.

Additionally, Sr is contained in order to fine Si particles deposited in diecasting. Since the Si content is 9.0 mass % or less, when the Sr content is less than 0.005 mass %, the above fining effect is not obtained. In contrast, when the Sr content exceeds 0.02 mass %, the above fining effect is saturated and it is difficult to obtain further effects, and consequently, yield is deteriorated. Therefore, the Sr content is set to be 0.005 to 0.02 mass %.

EXAMPLES

In the following, the present invention will be explained in more detail by Examples.

Alloys having the compositions shown in Table 1 were dissolved at 720° C., respectively and then were deoxidized and degassed by molten metal treatment using Ar gas and flux. Next, under a vacuum condition at an internal pressure of a metal mold of 5 kPa and at molten metal temperature 700° C., the alloys were cast using a metal mold for plate-shaped diecasting having width of 100 mm, depth of 300 mm, and height of 5 mm, and plate-shaped diecasting products having each composition shown in Table 1 (Examples 1 to 5 and Comparative Examples 6 to 12 of the present invention) were thereby obtained. Here, temperature of the metal mold was 150° C. Subsequently, the above diecasting products were subjected to each heat treatment which was respectively suitable under the condition described in Table 1. TABLE 1 units: mass % Heating Condition Si Fe Cu Mn Mg Zn Ti Sr (only Aging) Example 1 7.5 0.13 0.14 0.46 0.25 0.016 200° C., 2 hours Example 2 8 0.19 0.14 0.36 0.23 0.017 200° C., 2 hours Example 3 8.2 0.19 0.14 0.36 0.23 0.017 200° C., 2 hours Example 4 8.6 0.17 0.13 0.49 0.23 0.015 200° C., 2 hours Example 5 9 0.14 0.15 0.35 0.24 0.015 200° C., 2 hours Comparative 7 0.14 0.15 0.41 0.24 0.012 200° C., 2 hours Example 6 Comparative 8.5 0.33 0.22 0.3 0.4 0.011 200° C., 2 hours Example 7 Comparative 9.5 0.16 0.16 0.5 0.3 0.013 180° C., 3 hours Example 8 Comparative 10 0.14 0.13 0.46 0.21 0.014 180° C., 3 hours Example 9 Comparative 10.5 0.07 0.01 0.62 0.26 0.07 0.06 0.018 180° C., 3 hours Example 10 Comparative 10.3 0.08 0.01 0.65 0.35 0.06 0.02 180° C., 3 hours Example 11 Comparative 10.1 0.07 0.01 0.62 0.2 0.06 0.18 180° C., 3 hours Example 12

Next, in connection with the plate-shaped diecasting products (Examples 1 to 5 and Comparative Examples 6 to 12 of the present invention), test pieces for a tensile test having sizes shown in FIG. 5 were cut out from the center of the product, and test pieces (plate thickness of 2.5 mm with U-shaped notch) for a Charpy impact test shown in JIS Z2242 were cut out. The test pieces were subjected to normal temperature tensile test using a 5-ton tensile testing machine and a Charpy impact test using a 5-kg-m Charpy impact testing machine. These results are shown in Table 2. TABLE 2 Tensile Charpy Strength Proof Stress Value MPa MPa Elongation % J/cm² Example 1 278 188 11.4 10.7 Example 2 284 190 10.3 9.3 Example 3 286 191 10.3 9.5 Example 4 289 192 10.3 8.9 Example 5 293 193 10 8.1 Comparative 236 164 10.9 9.3 Example 6 Comparative 297 210 4.9 1.9 Example 7 Comparative 301 197 8.5 7.1 Example 8 Comparative 283 196 7.6 5.9 Example 9 Comparative 292 189 7.7 5.6 Example 10 Comparative 310 201 5.8 3.4 Example 11 Comparative 235 165 9.2 8.2 Example 12

As is apparent from Table 2, each diecasting product of Examples 1 to 5 exhibited superior results about not only tensile strength, proof stress and elongation but also impact value to those of each diecasting product of Comparative Examples 6 to 12.

Furthermore, each diecasting product was subjected to an evaluation about weldability. The welding was carried out according to the model figure shown in FIG. 6. In FIG. 6, numeral reference B showed total throat thickness (minimum thickness in building up portion by overlaying), and numeral reference Bb showed blowhole thickness. In addition, as described in the same figure, each diecasting product of Examples 1 to 5 and Comparative Examples 6 to 12 was used for a top plate, and T1 thickness of the plate was 4 mm. On the other hand, A5052P-O product was used for a bottom plate, and T2 thickness of the plate was 3 mm. Under such conditions, overlapping plates were assembled by fillet welding, MIG welding was carried out at a contact pressure of 3 tons, current of 230 A and voltage of 23 V, using filler metal A5356, and strip-shaped test pieces having width of 25 mm were cut out from the center of weld bead. The test pieces were subjected to normal temperature tensile test using a 5-ton tensile testing machine, and weld strength in the time was measured. The results are shown in Table 3. Here, alloys of Comparative Examples 10 to 12 were equivalent to 365 alloy of the AA standard. In the following, the results of each diecasting product are shown in the both cases in which diecasting contained gas amount was 2 cc/100 g and in which the gas amount was 8 cc/100 g. TABLE 3 Throat Thickness mm Welding Strength N Contained Gas Contained Gas Amount Contained Contained Amount 2 cc/100 g 8 cc/100 g Gas Gas Total Blowhole Total Blowhole Amount Amount Throat Thickness Throat Thickness 2 cc/100 g 8 cc/100 g Thickness B Bb Thickness B Bb Example 1 8779 8631 3.9 0 4.0 0.8 Example 2 9075 8898 4.2 0 4.2 0.9 Example 3 9114 8967 4.3 0 4.4 0.9 Example 4 9227 9103 4.4 0 4.5 0.9 Example 5 9364 9182 4.4 0 4.4 1.0 Comparative 7529 7442 3.9 0 3.9 0.9 Example 6 Comparative 9472 9315 4.3 0 4.3 1.2 Example 7 Comparative 8134 7646 3.7 0.1 3.9 1.7 Example 8 Comparative 7193 6759 3.2 0.3 3.7 1.7 Example 9 Comparative 7340 6826 3.3 0.2 3.8 1.9 Example 10 Comparative 7396 7026 3.3 0.2 3.7 1.8 Example 11 Comparative 6125 5728 3.2 0.3 3.9 1.9 Example 12

According to in Table 3, the results in the case in which diecasting products of Examples 1 to 5 were used, were generally superior to those in the case in which diecasting products of Comparative Examples 6 to 12 were used. This applied to not only when diecasting contained gas amount was 2 cc/100 g but also when it was 8 cc/100 g. Here, the results in the case in which diecasting contained gas amount was 2 cc/100 g, were generally superior to those in the case in which diecasting contained gas amount was 8 cc/100 g.

In addition, cross section of the bead portion of welding remaining material which adjoined the overlapped fillet welding test piece was water-polished by abrasive paper and was diamond-polished, and subsequently, the cross section of weld bead was observed. Specifically, throat thickness B (mm) and blowhole thickness Bb (mm) in the case in which the contained gas amount were different (2 cc/100 g and 8 cc/100 g) were measured at positions shown in FIG. 6. The results are shown in Table 3.

As is apparent from Table 3, in the weld bead using the diecasting products of Examples 1 to 5, superior weldability was exhibited because throat thickness was larger and blowhole thickness was smaller than those in the case in the weld bead using the diecasting products of Comparative Examples 6 to 12. Additionally, in the case in which the contained gas amount was 2 cc/100 g, if the diecasting products of Examples 1 to 5 were used, blowholes did not form and stabilized weldability was obtained.

In Si—Al based alloy of the present invention, in the case in which diecasting is carried out under high vacuum condition, weldability can be improved without increasing plate thickness of welding portion or reducing contained gas amount in diecasting. Therefore, the present invention is preferable to use as a various member in which more superior weldability will be required in the future. 

1. An Al—Si based alloy including Si at 7.5 to 9.0 mass %, Mg at 0.2 to 0.4 mass %, Mn at 0.3 to 0.5 mass %, Cu at 0.03 to 0.2 mass %, Fe at 0.1 to 0.25 mass %, Sr at 0.005 to 0.02 mass %, and the balance consisting of Al and inevitable impurities.
 2. The Al—Si based alloy according to claim 1, wherein Si is 7.5 to 8.5 mass %, Mg is 0.2 to 0.3 mass %, and Mn is 0.3 to 0.4 mass %.
 3. An alloy member subject made of the Al—Si based alloy according to claim 1 or
 2. 